The Whistlers in the Dark by Victoria Williamson
Scotland, 158 AD, is a divided country. On one side of the Antonine Wall, thirteen-year-old Felix is trying to become a good Roman soldier like his father. On the other, twelve-year old Jinny is vowing revenge on the 'metal men' who have invaded her Damnonii tribe's homeland. At the Damnonii's sacred circle of standing stones, her planned attack on Felix goes badly wrong, awakening a legend that threatens to bring fire and destruction down on them all. Can Jinny and Felix overcome their differences and soothe the stones back to sleep before it's too late?